This invention relates to a continuous process for preparing composite laminar membranes including a flexible microporous support and at least one ultrathin permeable nonporous polymeric membrane of 500 angstroms or less in thickness.
Methods employing liquid casting substrates for preparing thin semipermeable membranes of a variety of polymeric materials are known in the art. A method for solvent casting of ultrathin nonporous membranes on a liquid substrate is described in copending U.S. patent application Ser. No. 536,650 (Ward) filed Dec. 26, 1974, assigned to the assignee hereof, and incorporated herein by reference. Although as described therein relatively large (e.g., one square foot or more in area) ultrathin films may be prepared by the Ward method, the area thereof is limited by the area of the liquid casting substrate employed. Moreover, as taught therein, composites of one or more ultrathin membranes on a microporous backing layer are prepared using a vacuum pickup method wherein a differential pressure is applied across the ultrathin membranes during composite formation. Other batch casting methods employing liquid casting surfaces are described by Cadotte et al. in U.S. Pat. No. 3,580,841 and by Pate and Yaffe in Canad. J. Chem., Vol. 33, pp. 15-23 (1955). The present invention provides significant improvements over the method of the Ward application in that (1) the area of the composite membrane is not limited by the liquid surface area, (2) the films need not be subjected to applied differential pressure for application to the microporous backing, and (3) the composite laminar membrane is formed continuously.
In U.S. Pat. No. 3,767,737, Lundstrom teaches a process for producing thin polymer membranes, which includes continuously transferring a polymer-containing casting solution upwardly (employing, e.g., a roll) through a support liquid floating thereon for deposition on the upper surface of the liquid. After desolvation, the resulting continuous membrane is picked up on a flexible support passed under an osculation roller. The thickness of membranes provided from a polymer including alternating blocks of repeating units comprising bisphenol-A carbonate units and dimethylsiloxane units in this method is described in the patent as between about 0.005 mil (1,300 angstroms) and 0.05 mil (13,000 angstroms). Although the Lundstrom method represents a substantial advance in the art, the present invention provides significant improvements thereover in that (1) a solution transfer roll and the attendant power consumption are not required; (2) substantially thinner membranes can be prepared; (3) essentially simultaneously two ultrathin membranes may be formed and picked up on a backing layer in a single operation; (4) there is less risk of forming membranes with localized wet spots and variations in thickness; (5) there is less risk of loss of polymer through buildup on solid surfaces; and (6) there is less risk of disturbance of the liquid casting surface.
Larson et al., U.S. Pat. No. 3,933,561, describes a semi-continuous process for casting polymer films of thickness less than 25,000 angstroms wherein a solution of film-forming material is passed down an inclined surface under a thickness-control blade to a liquid casting surface. The film is continuously laminated to a release substrate. No blade is required in the present invention.
Thin films formed of organopolysiloxane-polycarbonate (PSPC) interpolymer and blends or mixtures thereof with polyphenylene oxide (PPO) as well as laminar composites of such films supported on a microporous backing layer are described in U.S. Pat. Nos. 3,980,456 (Browall) and 3,874,986 (Browall et al.) The Browall et al. U.S. Pat. No. 3,874,986 discloses use of a PSPC layer as an adhesive cushion intermediate the backing and a PPO-PSPC layer, while the Browall U.S. Pat. No. 3,980,456 describes use of a PSPC layer as an overcoat pinhole sealing membrane. The foregoing patents are incorporated herein by reference.
It has now been found by practice of the present invention that composite laminar membranes including a microporous backing layer and an ultrathin nonporous membrane of 500 angstroms or less in thickness can be prepared in continuous manner. It has further been found that such membranes additionally including a second such ultrathin membrane can be prepared continuously in a single operation.